A process of manufacturing a semiconductor device formed from a micropattern, such as an LSI or VLSI, adopts a reduction projection exposure apparatus, which reduces a pattern formed on a mask and projects and transfers it onto a substrate coated with a photosensitive agent. To increase the degree of integration of a semiconductor device, further micropatterning is required. The exposure apparatus takes a measure to realize micropatterning at the same time as the development of a resist process.
As a means for increasing the resolving power of an exposure apparatus, a method of shortening the wavelength of exposure light and a method of increasing the numerical aperture (NA) of a projection optical system are generally used. As for the wavelength of exposure light, a shift from a 365 nm i line to KrF excimer laser light having an oscillation wavelength of around 248 nm is in progress, and an ArF excimer laser, which emits light having an oscillation wavelength of around 193 nm, is being developed. A fluorine (F2) excimer laser, which emits light having an oscillation wavelength of around 157 nm, is also under development.
On the other hand, a projection exposure method using a liquid immersion method or liquid immersion lithography as a technique for increasing the resolving power independently of the above methods is receiving a great deal of attention. Conventionally, the space between the end face (final surface) of a projection optical system and the surface of an exposure target substrate (e.g., a wafer) is filled with a gas. However, the liquid immersion method executes projection exposure while the space is filled with a liquid. The liquid immersion method can advantageously attain a resolution higher than that in the conventional methods. For example, assume that the liquid immersion method uses pure water (refractive index:1.33) as the liquid to be supplied to the space between the projection optical system and the wafer, and sets the maximum incident angle of a light beam imaged on the wafer equal to that in the conventional methods. In this case, the liquid immersion method increases the resolving power to 1.33 times that in the conventional methods, even when the wavelength of light emitted from a light source remains the same. This amounts to increasing the NA of the projection optical system in the conventional methods to 1.33 times. The liquid immersion method can attain a resolving power whose NA is one or more, which is practically impossible in the conventional methods.
Methods of filling, with a liquid, the space between the wafer surface and the end face of the projection optical system are roughly classified into two. Japanese Patent Laid-Open No. 06-124873 discloses an exposure apparatus using the first method, i.e., a method in which the end face of the projection optical system and the entire wafer are arranged in a liquid tank. WO 99/49504 and Japanese Patent Laid-Open No. 2004-165666 disclose an exposure apparatus using the second method, i.e., a local fill method, in which the liquid is supplied only to the space between the projection optical system and the wafer surface.
As described above, the liquid immersion method executes projection exposure by filling, with the liquid, the space between the end face of the projection optical system and the surface of the exposure target substrate (e.g., a wafer). This makes it possible to attain a resolving power whose NA is one or more, which is practically impossible in the conventional methods. If pure water is used as the liquid, the liquid immersion method can attain a resolving power 1.33 times that in the conventional methods, because the refractive index of pure water is 1.33. If, however, a liquid having a refractive index higher than that of pure water is used, it is possible to attain a resolution higher than that in the use of pure water. Unfortunately, most of currently available liquids having a high refractive index are substances having a property of corroding a substrate and a stage, although they do not contaminate optical components, or substances having a property of contaminating the optical components, although they do not corrode the substrate and the stage. Therefore, it is problematic to use such a liquid as the immersion liquid.